1. Field of the Invention
The present invention relates to a method for forming plasma-sprayed thermal barrier coatings over the surfaces of the metal bodies such as the combustor transition pieces, turbine rotor blades, and turbine stator blades as to the industrial gas turbines.
2. Background of the Invention
Gas turbines are used for the emergency power generating facilities, as the gas turbines need neither cooling water nor long start-up time; gas turbines are used for the combined cycle power plants (gas-turbine steam-turbine combined cycle power plants) of a large scale because of the high efficiency of the combined cycle power generation.
The gas turbine is a centrifugal, axial or radial turbo machine that includes three major configuration parts, namely, a compressor, a combustor, and a turbine. In the gas turbine, the air compressed by the compressor is supplied to the combustor(s) in which the fuel is injected so as to be burnt; thereby, the combustion gas of a high temperature and a high pressure is generated; and, the combustion gas flows into the centrifugal, axial or radial turbine so as to drive the gas turbine (so as to make the gas turbine rotate). In general, the turbine is directly connected (without gear connections) to the compressor, transferring the power needed for compressing the air to be supplied to the compressor.
In order to improve the efficiency of the gas turbine, it is desirable to enhance the turbine inlet temperature (TIT); thus, TIT has been increased in the field of the gas turbine. The TIT for the gas turbines operated in the actual thermal power plants is usually at a level within a range around from 1300 to 1500° C.
The parts that form the combustor, the combustor transition piece that guides the high temperature/pressure combustion gas from the combustor to the turbine, the turbine rotor blades, and the turbine stator blades are exposed to the combustion gas of the temperature around from 1300 to 1500° C.; these gas turbine components are provided with the thermal barrier coatings (often abbreviated as TBC) so as to achieve high durability. For instance, the patent reference 1 (JP patent 2977369) discloses a rotor or stator blade with the surface TBC comprising a first layer that is made of NiCrAlY (nickel.chrome.aluminum.yttrium) alloy or CoNiCrAlY (cobalt.nickel.chrome.aluminum.yttrium) alloy, the layer being formed by means of the low pressure plasma spray coating; a second layer that is made of ZrO2-Y2O3 material, the layer being formed by means of the atmospheric pressure plasma spray coating; a third layer that is made of fine ceramics and forms oxygen-permeable layer, the layer being formed by means of the chemical vapor deposition or the low pressure plasma spray coating.
In forming the TBC on the parts configuring the gas turbine as described in the above, a robot comprising a plasma spray gun is used in order that the coating material is sprayed from the spray gun toward the to-be-coated surface or the whole surface of the to-be-coated part in response to the predetermined plasma spray conditions, while the robot is moved toward a predetermined direction at a predetermined speed. The referred plasma spray conditions depends on the shape, the to-be-coated part material and so on; thus, before performing the spray coating by use of the robot, it becomes necessary to instill (teach) how to spray plasma coating in (to) the robot. It is hereby noted that the term “(robot) teaching” mean to teach the robot how to move and work hereafter in this specification.
The conventional robot teaching for establishing the plasma spray coating conditions is a manner in which a test plasma spray coating is performed to a to-be-spray-coated part, and the inspection of the coated part is executed, on the premise that the part is inexpensive; namely, if the inspection result is negative (not satisfactory), the same process (modified coating test on an equivalent part) is repeated until the inspection result becomes satisfactory. In other words, the tested parts until the inspection result becomes satisfactory are thrown away.
In a case where the to-be-spray-coated part is expensive, the robot teaching method in which the throwaway practice as described above is incorporated is not feasible from the economical point of view; in fact, the above referred parts such as the combustor transition pieces, turbine rotor blades, and turbine stator blades are the examples of expensive parts. In particular, the combustor transition pieces are made of the expensive Ni-base alloy as the patent reference 2 (JP patent 3067416) discloses; further, the transition pieces are provided with a plurality of fine through-holes for cooling the combustion gas flow film (boundary layer), the fine holes being not easily machined; and, the manufacturing cost of the combustor transition pieces currently reach several millions yen per gas turbine. Therefor, the robot teaching method in which the throwaway practice is not feasible at all.
Thus, in the conventional robot teaching (method) for performing the plasma spray coating, the inner surface of the combustor transition pieces is masked with double or triple layers of tape so that foreign substances do not clog the fine through-holes; then, on the layers of tape, the test specimens are paved with a space of approximately five centimeters between a piece (specimen) and the adjacent piece (specimen), the test specimens being made of the same material as that of the combustor transition pieces; further, the trailer parts (end edge areas) of the test specimens are fixed to the layers of tape (the masking tape), by use of the tape of the same material as that of the layer tape (the masking tape); then, the plasma spray coating test is performed so as to execute the robot teaching.
In addition, the tape can be, for example, PTFE tape that is made of fiber glass impregnated with polytetrafluoroethylene resin, one side of the tape having an adhesive coating of silicon (silicon-base material); or, the tape can be the tape comprising silicon rubber, aluminum foil and fiber glass, the tape material being able to be used for the plasma spraying.
Further, the patent reference 3 (JP1993-111666) discloses a masking method for forming a (hard) resist film on the to-be-plasma-splayed area on which the hardened film can be formed by use of a method such as photo-curing or heat curing, the film being made of resin that is able to be resistant against plasma spraying (heat) as well as to be removed after plasma alloy spraying, the resin being applied or printed on the to-be-plasma-splayed area in a liquid condition, dried on the area and hardened by light or heat.
As for the above-described robot teaching (method) by use of the test specimens fixed on the to-be-plasma-sprayed (metal) part with the layers of tape, the plasma spraying heat sometimes scorch the tape in the test plasma spraying; thus, the surface of the metal part is exposed and plasma material clogs the fine through-holes of the tested part. Further, the heat scorches the tape fixing the pieces so that the test specimens sometimes move from the predetermined positions or the TBC plasma spray reaches the backside of the tested part. Moreover, the masking method by use of the tape is so difficult that even skilled craftsmen need a lot of man-hours to perform the method.
Even if the masking method accompanies the approach for forming a (hard) resist film on the to-be-plasma-splayed area of the inner surface of the transition piece as disclosed by the patent reference 3, it is necessary to use the tape to fix the test specimens; thus, the problem that the tape may be scorched remains unsolved.